Framerate drops significantly when attempting to import heightmaps

Framerate drops significantly when attempting to import heightmaps. Once the heightmap is imported this error is no longer prevalent, but during the preview there is a ~80-100 FPS drop.

Regression (yes) issue (does not) occur in 4.12.5

1. Download attached heightmaps
2. Open Editor (any project)
3. Console Command 'Stat FPS'
4. Modes>Landscape
5. Press Import from Heightmap
6. Select "..." next to import from file
7. Select either heightmap

Results

Framerate drops from 120 to 3 (for 2048x2048 heightmap) or 20 (for 1024x1024 heightmap) while heightmaps is within the wireframe preview. Once imported FPS returns to normal

Expected

Heightmap appears in preview, no noticable FPS loss or minimal FPS loss.

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